It is often desirable to determine the capacity of a battery, usually measured in amp-hours (Ah), for reliable operation of battery powered devices. One conventional method for determining the capacity of a battery, for example, a lead-acid battery, requires fully charging the battery to 100% state of charge (SOC) and then fully discharging the battery at a constant current. The battery capacity is determined by multiplying the discharge current by the discharge time needed to fully discharge the battery. Following the discharge to determine its capacity, the battery must be fully charged again to be ready for use. This conventional method is time consuming and may be destructive of the battery's health, as gassing may occur in overcharging the battery and cause damage.
To address the need to charge and discharge the battery to determine capacity, another conventional method uses a correlation of Iup to capacity. Iup is the local maximum of the slope of the battery terminal voltage in response to the increasing portion of a current ramp. Iup corresponds to the transition from a charge reaction to an overcharge reaction. In lead-acid batteries, the overcharge reaction is generation of hydrogen and oxygen gas, 2H2O2H2+O2. For example, a linearly increasing current ramp is applied to a lead-acid battery while measuring the battery terminal voltage. Upon reaching a predetermined voltage safe limit, a linearly decreasing current ramp is applied while measuring battery terminal voltage. The slope of the voltage response is then calculated and the local maximums determined. The local maximum during the increasing portion of the current ramp Iup is then correlated to battery capacity.
This conventional method is limited, however, to use on lead-acid batteries having a full charge specific gravity (SG) of 1.28. Specific gravity of the battery refers to a ratio of the density of the battery's electrolyte to the density of water. Problems arise because recently introduced lead-acid batteries have a full charge specific gravity of 1.30. Because the conventional methods and apparatus for determining capacity cannot distinguish between the lower specific gravity batteries (1.28 SG) and the higher specific gravity batteries (1.30 SG), incorrect capacity determination results when using the higher specific gravity batteries with conventional methods and apparatus.
Thus, there is a need to overcome these and other problems of the prior art and to provide better methodology and apparatus to determine battery capacity.